Plant basal resistance - Universiteit Utrecht

Chapter 1
Selective non-pathogenic root-colonizing microbes can induce systemic defence
priming as well (Van Wees et al., 2008). The resulting disease resistance is commonly
referred to as induced systemic resistance (ISR). For instance, non-pathogenic rhizobacteria
can prime Arabidopsis against a wide range of plant pathogens, like bacteria, oomycetes,
fungi, viruses and even herbivores (Pieterse et al., 1996; Ton et al., 2002; Van Wees et al.,
2008). A more recent example by (Verhagen et al., 2010) showed that beneficial bacteria
such as Pseudomonas fluorescens CHA0 and Pseudomonas aeruginosa 7NSK2 mediate
ISR in grapevine through potentiating oxidative burst and phytoalexin production (i.e.
resveratrol and viniferin) after attack by Botrytis cinerea. Arbuscular micorhizal fungi (AMF)
can also induce defence priming in plants (Pozo and Azcón-Aguilar, 2007; Pozo et al., 2009).
The interaction between Arabidopsis and Pseudomonas fluorescens WCS417r has served
as a biological model system to study the signalling transduction pathways controlling ISR.
Research on this model system revealed P. fluorescens WCS417r–mediated ISR in Arabidopsis
requires responsiveness to jasmonate and ethylene (ET) and is dependent on NPR1 (Pieterse
et al., 1998). A transcriptome analysis for P. fluorescens WCS417r-inducible genes led to the
identification of the ISR responsive transcription factor gene MYB72 (Verhagen et al., 2004).
Subsequent analysis of mutant lines in MYB72 revealed that this transcription factor gene
plays a critical role for the early signalling events leading to elicitation of the systemic signal
(Van der Ent et al., 2008). The same authors reported that P. fluorescens WCS417r–mediated
priming in Arabidopsis thaliana against Hyaloperonospora arabidopsidis, an oomycete
pathogen that is unaffected by JA- and ET-dependent defences (Thomma et al., 1998; Ton et
al., 2002), is based on priming of callose deposition, which requires intact abscisic acid (ABA)
signalling (Van der Ent et al., 2009).
Apart from defence priming against pathogens, systemic defence priming can also
be effective against herbivore attack. When plants are subjected to damage by herbivorous
insects, they emit a complex blend of airborne chemical signals, known as volatile organic
compounds (VOCs). VOCs serve primarily to attract natural enemies of the herbivore
(Turlings and Ton, 2006), but they have also been shown to induce defence priming in
systemic tissues and even neighbouring plants (Engelberth et al., 2004; Heil and Silva
Bueno, 2007; Ton et al., 2007; Heil and Ton, 2008). Three green leaf volatiles, (Z)-3-hexenal,
(Z)-3-hexen-1-ol, and (Z)-3-hexenyl acetate, in particular have been linked to elicitation of
defence priming by herbivore-induced VOCs (Engelberth et al., 2004). VOC-induced priming
augments JA-inducible defences (Frost et al., 2008). Interestingly, however, only a sub-set
of JA-dependent genes is responsive to priming by VOCs in maize (Ton et al., 2007). The
latter observation suggests that priming-inducing VOCs target specific components in the JA
response pathway.
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